Feb 23 2015
Widespread Winds and Feedback from Supermassive Black Holes
Astronomers have discovered that the winds from supermassive black holes at the centre of galaxies blow outward in all directions, a suspected phenomenon that had been difficult to prove before now.
This plot of data from two space telescopes, NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's (ESA's) XMM-Newton determines for the first time the shape of ultra-fast winds from supermassive black holes, or quasars. The winds blow in every direction, in a nearly spherical fashion, coming from both sides of a galaxy (only one side is shown here). Image credit: NASA/JPL-Caltech/Keele Univ.
These new findings, by an international team of astrophysicists, were made possible by simultaneous observations of the luminous quasar PDS 456 with ESA's XMM-Newton and NASA's NuSTAR X-ray telescopes, and support the picture of black holes having a significant impact on star formation in their host galaxies.
At the core of every massive galaxy in the Universe, including our own Milky Way, sits a supermassive black hole, with a mass some millions or billions of times that of our Sun. Some of these black holes are active, meaning that their intense gravitational pull causes matter to spiral inward, and at the same time part of that matter is cast away through powerful winds.
"We now know that quasar winds significantly contribute to mass loss in a galaxy, driving out its supply of gas, which is fuel for star formation," said Dr Emanuele Nardini, of the X-ray Astrophysics Group at Keele University in the UK and lead author of the study. "This study provides a unique view of the possible mechanism that links the evolution of the central black holes to that of their host galaxies over cosmic time."
With the shape and extent of the winds determined, the researchers could then figure out their power and answer general questions about the degree to which they can quench the formation of new stars. Astronomers think that supermassive black holes and their galaxies co-evolve together, regulating each other's growth. For more than a decade astronomers have investigated the correlation between the mass of stars in the bulge of a galaxy and the mass of its central black hole, yet it is by no means obvious that the black hole could have an impact on its host galaxy as a whole.
“Black holes of this kind are very powerful, but their gravitational field only extends over the very inner parts of a galaxy,” explains Dr Nardini. “For black holes to really influence the star-forming activity of an entire galaxy, there must be a feedback mechanism connecting the two on a global scale.”
One possibility is that the propagation of winds driven by the black hole's accretion activity plays a role and, as reported in the journal Science, Dr Nardini and collaborators have obtained the first solid evidence supporting this scenario.
The researchers have looked at PDS 456, a galaxy that lies just over two billion light-years away and that hosts an exceptionally active black hole with a mass of one billion Suns. PDS 456 is a quasar, a class of galaxies that appear as a point source because the activity of the central black holes outshines the brightness produced by their stars. A comprehensive view of the accreting activity of this quasar could be obtained by observing PDS 456 simultaneously with ESA's XMM-Newton X-ray observatory and NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) mission. The observations were performed on four occasions in 2013, and once again in 2014.
Since the early X-ray detections, this quasar revealed a strong absorption feature caused by iron nuclei that have been stripped of all but one or two of their electrons. “The absorption line is blue-shifted with respect to its expected energy in the laboratory, indicating that it arises in a gas that is moving away from the black hole; in other words, we are seeing an outflowing wind,” explains Dr Nardini. This line has now been detected in many other quasars, but due to its relative vicinity to us, PDS 456 offers ideal conditions to observe it in detail.
Prior to this study, astronomers used this absorption feature to learn that the wind is blowing at one third the speed of light. But the data were not enough to determine the amount of matter and energy carried away from the black hole. This is where the new observing campaign made a difference, allowing the astronomers to uncover something new: not only they could see the absorption caused by the iron ions, now they also detected direct emission from those ions.
“It was a most welcome surprise,” says Dr Nardini.
The emission signature in the spectrum was detected at a slightly lower energy than the absorption feature, indicating that the emitting ions lie outside our line of sight.
“We are seeing material that is flowing away from the black hole, not only towards us but in every direction,” he adds.
Such a pair of emission and absorption lines is called a P-Cygni profile, as it was first observed in the variable star P Cygni. It is characteristic of a gaseous envelope expanding away from the central source, and it was never before observed in a wind launched from the vicinity of a supermassive black hole. With this new feature detected in the spectra of PDS 456, the astronomers could finally study the geometry of the wind blown by the black hole, revealing a wide, almost spherical outflow of matter.
“Knowing the speed, shape and size of the winds, we can now figure out how powerful they are,” comments Fiona Harrison of the California Institute of Technology in Pasadena, California, who is a co-author on the paper and the principal investigator of NuSTAR.
The data indicate that the outflowing material amounts to about ten times the mass of the Sun every year, and that the kinetic power it releases into the surroundings is about 20 per cent of the total energy emitted by the quasar.
The wide shape of the wind suggests that the black hole must have quite an impact on the host galaxy, and the estimated amounts of mass and energy that are being blown away seem to confirm that the outflow is able to trigger an effective feedback mechanism on the galaxy as a whole. While the black-hole wind consists of ionised gas, its power has the potential to set larger outflows into motion, eventually driving away the galaxy's reservoir of molecular gas – the raw material that is needed for stars to form.
Because PDS 456 is relatively nearby by cosmic standards, in this new study astronomers might be witnessing the early stage of a feedback process that more distant quasars underwent around 10 billion years ago, when supermassive black holes and their fierce winds were much more common and possibly contributed to regulating the star-forming activity of the galaxies we observe today.
“This is a great example of the synergy between XMM-Newton and NuSTAR,” says Norbert Schartel, XMM-Newton project scientist at ESA.
“The complementarity of these two X-ray observatories is enabling us to unveil previously hidden details about the powerful side of the Universe."